well, the real reason is, i never seen McHale's navy (i had to google what the significance of PT-73 was)
but a better reason is, from what i found when i googled it, the PT-73 used in the show was a 72 foot type II vosper, while the real PT-73 was a 78 foot Higgings...neither one is the exact same as the 80ft Elco model i am building